1. Field of The Invention
The present invention relates to methods of molding. More particularly, the invention relates to an improved method of injection molding of preforms.
2. Description of the Prior Art
Molding operations are commonly done with either reciprocating screw or two stage injection units. When an object is to be made by a blow molding process, such as a beverage container, it is common practice to make a parison or a preform as an initial stage of the process. It is common practice in the manufacture of such preforms to use a polyethylene therephthalate (PET) material to make such preforms. Such PET reheat and blow preforms commonly use injection molding equipment of the two stage injection type. This two stage injection system allows the plastication or extruder screw to focus on melting the material, while the shooting pot/plunger system injects the molding plastic into the injection mold.
This separation of functions allows optimization of each of the processes in a relatively independent manner. The major focus of optimization in the plastication or extrusion screw is the minimization of polymer degradation while maintaining melt output. The predominant degradation product generated during standard melt processing is acetaldehyde. Acetaldehyde has been demonstrated to affect the taste of bottled water, cola soft drinks, and other beverages at very low concentration levels.
Separating the plastication function from the injection function also allows the plastication screw to run in a nearly continuous manner, with the screw stopping momentarily just prior to melt transfer from the plastication unit to the injection unit. This mode of operation minimizes degradation of the molten polymer by reducing the amount of sheer heat generated during plastication through the use of reduced speeds. Also, because the screw is operating in a nearly continuous mode, the amount of degradation that occurs due to residence time in the melt is also reduced because the diameter of the screw, and thus its melt residence time, can be reduced when compared to a standard reciprocating screw injection unit.
In general, the two stage injection systems have had the ability to provide melt with acceptable levels of acetaldehyde. However, the acetaldehyde produced by the sheer heat in the plasticizing stage and residence time in the melt are not the only factors determining the acetaldehyde level. Other factors such as the particular material, the particular mix of the preferred material, the size of the shot and other factors are all important in determining the final level of acetaldehyde in the melt. When all of these factors combined, the prior art methods were found to meet specifications, but always on the high end of the specification level, making the process much harder to control. Thus, those skilled in the prior art continued to search for a solution to reduce the acetaldehyde content of the finished product which was traceable to the plastification and transfer function.